Method for selecting an interlayer for vibroacoustic damping, interlayer for vibroacoustic damping and glazing unit comprising such an interlayer

ABSTRACT

A method for selecting a viscoelastic plastic interlayer including two outer layers and a central layer. The interlayer is for incorporation between two glass sheets of a glazing unit. The method of selection includes providing first and second components respectively as the central layer and the outer layers, and measuring the shear modulus G′ of the first and second components. The material of the second component is selected only if G′ is a certain value at a specific temperature and frequency range. The thickness h of the first component is set so that h is a specific value and so that g=G′/h is within a specific range of values at a specific temperature and frequency range. The method optimizes the damping of the second and third natural frequencies of a vehicle windshield without weighing down the windshield.

The invention relates to a method for selecting an interlayer having an acoustic damping property in order to be incorporated into a laminated glazing unit, intended in particular for vehicles of locomotion, in particular a motor vehicle.

Among all the qualities contributing to comfort in modern means of transport such as trains and motor vehicles, silence has become the determining factor.

Acoustic comfort has been improved now over several years, by dealing with noise, such as noise from the engine, tires or suspension, and this at their source or during their propagation through the air or solids, by means for example of absorbent coatings or elastomeric connecting components.

The shapes of vehicles have also been modified in order to improve penetration through the air and to reduce turbulence that is itself a source of noise.

For several years, emphasis has been given to the role that glazing units can play in improving acoustic comfort, in particular laminated glazing units comprising plastic interlayer films. Laminated glazing units have, in addition, other advantages such as eliminating the risk of flying fragments in the case of sudden breakage, and impeding break-ins.

It has been demonstrated that the use of standard plastic films in laminated glazing units is not suitable for improving acoustic comfort. Specific plastic films have thus been developed that have damping properties enabling acoustic comfort to be improved.

In the following description, reference to a damping film relates to a viscoelastic plastic film that provides improved damping of vibrations in order to give the glazing unit a function of noise reduction.

It has been shown that the acoustic performance of a glazing unit depends on the value of the loss factor tan δ of the material constituting the interlayer film. The loss factor is the ratio between the energy dissipated in the form of heat and the elastic strain energy; it characterizes the capacity of the material to dissipate energy. The higher the loss factor, the greater the energy dissipated and therefore the more the material plays its damping role.

This loss factor varies as a function of the temperature and of the frequency. For a given frequency, the loss factor reaches its maximum value at a temperature known as the glass transition temperature.

The materials used as interlayers of laminated glazing units are viscoelastic plastic films, of acrylic polymer or acetal resin or else polyurethane type for example, which have quite a high loss factor, such as at least greater than 0.6 for a given temperature range and for a given frequency range.

The loss factor tan δ is assessed using a viscoanalyzer. The viscoanalyzer is a machine that makes it possible to subject a specimen of material to strains under precise temperature and frequency conditions, and thus to obtain and process all of the rheological magnitudes that characterize the material.

It has also been described, regarding the integration of a damping interlayer in a laminated glazing unit, that the loss factor tan δ should not be considered alone but that the shear modulus G′ constitutes another characteristic to be taken into account in the damping property of the interlayer. Document EP-A-844 075 teaches that, in order to damp vibrations, the interlayer of the laminated glazing unit must correspond to particular values as regards the shear modulus G′ and the loss factor tan δ. It is recalled that the shear modulus G′ characterizes the stiffness of the material; the higher G′, the stiffer the material, and the lower G′, the more flexible the material. The shear modulus depends on the temperature and on the frequency. The shear modulus G′ is also assessed using a viscoanalyzer. This document describes that the loss factor tan δ of the interlayer is greater than 0.6 and the shear modulus G′ of the interlayer is less than 2×10⁷ Pa for temperatures between 10° C. and 60° C. and frequencies between 50 Hz and 10 000 Hz, in order to damp, more particularly, noises of solid-borne origin.

Furthermore, when the laminated glazing unit is used as a windshield, it is subjected to acoustic vibrations that are specific to it. Thus, the four first natural frequencies of the windshield, and in particular the second and third natural frequencies of the windshield, between 100 Hz and 240 Hz, are particularly annoying, acoustically speaking. The interlayer of document EP-A-844 075 is suitable for damping solid-borne noises, but not for the vibroacoustic damping of the first natural frequencies of the windshield, in particular the second and third natural frequencies.

There is therefore a need for a method for selecting an interlayer which makes it possible to optimize the damping of the first natural frequencies of a windshield, in particular of the second and third natural frequencies of the windshield, without weighing down the windshield.

For this, the invention proposes a method for selecting a viscoelastic plastic interlayer comprising two outer layers and a central layer and intended to be incorporated between two sheets of glass of a glazing unit, the method comprising the following steps:

-   -   a first component made of a viscoelastic plastic material         intended to constitute the central layer and a second component         made of a viscoelastic plastic material intended to constitute         the outer layers are provided,     -   the shear modulus G′ of the first component and of the second         component are measured by means of a viscoanalyzer,     -   the material of the second component is only selected if its         shear modulus G′ is greater than or equal to 3×10⁷ Pa at 20° C.         and for a frequency range between 100 Hz and 240 Hz; and     -   the thickness h of the first component is set so that h is less         than or equal to 0.3 mm and so that the shear parameter g=G′/h,         G′ being the shear modulus, is between 8×10⁸ Pa/m and 2.67×10⁹         Pa/m at 20° C. and for a frequency range between 100 Hz and 240         Hz.

According to another feature, the thickness h of the first component is set so that h is less than or equal to 0.1 mm and so that G′/h is between 1.33×10⁹ Pa/m and 2×10⁹ Pa/m at 20° C. and for a frequency range between 100 Hz and 240 Hz.

According to another feature, the method comprises, in addition, before the step of setting the thickness h, the following steps:

-   -   the loss factor tan δ of the first component is measured by         means of the viscoanalyzer; and     -   the first component is selected only if its loss factor tan δ is         greater than 0.6.

According to another feature, the method comprises, in addition, the following step:

-   -   the material of the second component is selected only if its         shear modulus G′ is between 10⁸ Pa and 2×10⁸ Pa at 20° C. and         for a frequency range between 100 Hz and 240 Hz.

According to another feature, the method comprises, in addition, the following steps:

-   -   it is verified that the adhesion of the material of the second         component is compatible with the requirements of Regulation R43         by twisting a specimen of the interlayer constituted of the         material of the second component bonded to two glass sheets, by         measuring the torsional force at which separation of the         interlayer constituted of the material of the second component         with the glass sheets is initiated, and by calculating from this         force the corresponding adhesive shear strength, and this value         of the adhesive strength is compared with a range of admissible         values so that any laminated glazing unit withstands the         stresses corresponding to Regulation R43; and     -   the thickness e of the second component is set in the following         manner:         -   a reference laminated glazing unit is identified that             withstands the stresses corresponding to Regulation R43 and             which comprises two sheets of glass and an interlayer             constituted of the material of the second component;         -   the tear strength of the interlayer of the reference             laminated glazing unit, the thickness of the interlayer of             the reference laminated glazing unit and the thickness of             the glass sheets of the reference laminated glazing unit are             determined;         -   using a graph representative of the minimum interlayer tear             strength required so that any laminated glazing unit             withstands the stresses corresponding to Regulation R43, as             a function of the interlayer thickness of any laminated             glazing unit, this graph being established for a substrate             thickness of any laminated glazing unit equal to the             substrate thickness in the reference laminated glazing unit,             the minimum required interlayer thickness is deduced which             corresponds to a minimum required interlayer tear strength             value equal to the tear strength of the interlayer of the             reference laminated glazing unit; and         -   the thickness e of the second component is set so that e is             greater than or equal to said optimum interlayer thickness             value.

There is also a need for an interlayer which makes it possible to optimize the damping of the first natural frequencies of a windshield, in particular of the second and third natural frequencies of the windshield, without weighing down the windshield.

For this, the invention proposes a viscoelastic plastic interlayer intended to be incorporated between two glass sheets of a glazing unit in order to give it vibroacoustic damping properties, the interlayer comprising:

-   -   two outer layers made of a viscoelastic plastic material, the         shear modulus G′ of which is greater than or equal to 3×10⁷ Pa         at 20° C. and for a frequency range between 100 Hz and 240 Hz,     -   a central layer having vibroacoustic damping properties, of         thickness h, so that h is less than or equal to 0.3 mm and so         that the shear parameter g=G′/h of the central layer, G′ being         the shear modulus, is between 8×10⁸ Pa/m and 2.67×10⁹ Pa/m at         20° C. and for a frequency range between 100 Hz and 240 Hz,

the central layer being between the two outer layers.

According to another feature, the thickness h of the central layer is such that h is less than or equal to 0.1 mm and such that G′/h is between 1.33×10⁹ Pa/m and 2×10⁹ Pa/m at 20° C. and for a frequency range between 100 Hz and 240 Hz.

According to another feature, the central layer has a loss factor tan δ greater than 0.6.

According to another feature, the outer layers have a shear modulus G′ between 10⁸ Pa and 2×10⁸ Pa at 20° C. and for a frequency range between 100 Hz and 240 Hz.

According to another feature, each of the outer layers has a thickness e such that:

-   -   the adhesion of the material of the outer layers is compatible         with the requirements of Regulation R43, the adhesion being         determined by twisting a specimen of interlayer constituted of         the material of the outer layers bonded to two glass sheets, by         measuring the torsional force for which the separation of the         interlayer constituted of the material of the outer layers from         the glass sheets is initiated, and by calculating, from this         force, the corresponding adhesive shear strength, then by         comparing this value of the adhesive strength to a range of         admissible values so that any laminated glazing unit withstands         the stresses corresponding to Regulation R43; and such that     -   the thickness e of each of the outer layers is set so as to         satisfy the requirements of Regulation R43, the thickness e         being determined in the following manner:         -   a reference laminated glazing unit is identified which             withstands the stresses corresponding to Regulation R43 and             which comprises two glass sheets and an interlayer             constituted of the material of the outer layers;         -   the tear strength of the interlayer of the reference             laminated glazing unit, the thickness of the interlayer of             the reference laminated glazing unit and the thickness of             the glass sheets of the reference laminated glazing unit are             determined;         -   using a graph representative of the minimum interlayer tear             strength required so that any laminated glazing unit             withstands the stresses corresponding to Regulation R43, as             a function of the interlayer thickness of any laminated             glazing unit, this graph being established for a substrate             thickness of any laminated glazing unit equal to the             substrate thickness in the reference laminated glazing unit,             the minimum required interlayer thickness is deduced which             corresponds to a minimum required interlayer tear strength             value equal to the tear strength of the interlayer of the             reference laminated glazing unit; and         -   the thickness e of each outer layer is set so that e is             greater than or equal to said optimum interlayer thickness             value.

According to another feature, the central layer comprises:

-   -   a damping film made of a viscoelastic plastic material A of         thickness h_(A) and with a shear parameter g_(A);     -   a damping film made of a viscoelastic plastic material B of         thickness h_(B) and with a shear parameter g_(B),     -   the materials A and B each having, over a respective temperature         range t_(A) and t_(B) and for a frequency between 100 Hz and 240         Hz, a loss factor greater than 0.6 and a shear parameter between         8×10⁸ Pa/m and 2.67×10⁹ Pa/m, and the film which has the highest         loss factor, for a given range of temperatures included within         the temperature range t_(A) or t_(B) respectively, has an         equivalent shear parameter g_(A or B eq)=g_(A or B)×h,         g_(A or B) being the shear parameter of the material         constituting the film and h being the thickness of the central         layer, which is less than the equivalent shear parameter of the         other film for said temperature range.

The invention also relates to a glazing unit comprising:

-   -   a glass sheet having a thickness between 1.4 mm and 2.1 mm,     -   a glass sheet having a thickness between 1.1 mm and 1.6 mm, and     -   an interlayer described above, the interlayer being between the         glass sheets.

According to another feature, the thickness e of each of the outer layers and the total thickness of the glass sheets are such that:

-   -   the adhesion of the material of the outer layers is compatible         with the requirements of Regulation R43, the adhesion being         determined by twisting a specimen of interlayer constituted of         the material of the outer layers bonded to two glass sheets, by         measuring the torsional force for which the separation of the         interlayer constituted of the material of the outer layers from         the glass sheets is initiated, and by calculating, from this         force, the corresponding adhesive shear strength, then by         comparing this value of the adhesive strength to a range of         admissible values so that any laminated glazing unit withstands         the stresses corresponding to Regulation R43; and such that     -   the thickness e of each of the outer layers and the total         thickness of the glass sheets are set so as to satisfy the         requirements of Regulation R43; they are determined in the         following manner:         -   a reference laminated glazing unit is identified which             withstands the stresses corresponding to Regulation R43 and             which comprises two glass sheets and an interlayer             constituted of the material of the outer layers;         -   the tear strength of the interlayer of the reference             laminated glazing unit, the thickness of the interlayer of             the reference laminated glazing unit and the thickness of             the glass sheets of the reference laminated glazing unit are             determined;         -   using a graph representative of the minimum interlayer tear             strength required so that any laminated glazing unit             withstands the stresses corresponding to Regulation R43, as             a function of the interlayer thickness of any laminated             glazing unit, and of the thickness of glass sheets of any             laminated glazing unit, a combination of optimum values of             the interlayer thickness and of the thickness of glass             sheets is deduced which corresponds to a minimum required             interlayer tear strength value equal to the tear strength of             the interlayer of the reference laminated glazing unit; and         -   the thickness e of each outer layer is set so that e is             greater than or equal to said optimum interlayer thickness             value and the thickness of the glass sheets is set greater             than or equal to said optimum glass sheet thickness value.

The invention also relates to a vehicle comprising the glazing unit as described above, the glass sheet having a thickness between 1.4 mm and 2.1 mm being turned toward the outside of the vehicle and the glass sheet having a thickness between 1.1 mm and 1.6 mm being turned toward the inside of the vehicle.

The invention also relates to the use of the interlayer as described above for the vibroacoustic damping of the second and third natural frequencies of a windshield constituted of two glass sheets and the interlayer incorporated between the glass sheets.

The invention also relates to the use of the glazing unit described above as a vehicle windshield.

Other features and advantages of the invention will now be described with respect to the drawings, in which:

FIG. 1 represents a curve of the modal damping of a laminated windshield as a function of the shear parameter of a central layer of the interlayer of the laminated windshield for frequencies between 100 Hz and 240 Hz at 20° C.;

FIG. 2 represents a cross-sectional view of a glazing unit according to the invention;

FIG. 3 represents a cross-sectional view of a glazing unit according to the invention, according to another embodiment;

FIG. 4 represents a graph that is representative of the minimum interlayer tear strength required in order for a laminated glazing unit to withstand the stresses corresponding to Regulation R43, as a function of the interlayer thickness of the laminated glazing unit, established for a laminated glazing unit comprising two glass substrates having a thickness of 2.1 mm and an interlayer having an adhesive strength between 2 MPa and 5 MPa and for a drop height of 4 m;

FIG. 5 represents a three-dimensional graph, which is representative of the minimum interlayer tear strength required in order for a laminated glazing unit to withstand the stresses corresponding to Regulation R43, as a function both of the interlayer thickness and of the glass thickness of the laminated glazing unit, established for a laminated glazing unit comprising an interlayer having an adhesive strength between 2 MPa and 5 MPa and for a drop height of 4 m;

FIG. 6 represents a schematic front view of an experimental device for evaluating the adhesion of an interlayer with respect to glass sheets with which it is combined;

FIG. 7 represents a perspective view of a variant of a device for evaluating the adhesion of an interlayer with respect to glass sheets with which it is combined; and

FIG. 8 represents a schematic view of an experimental device for evaluating the tear strength of an interlayer.

The reference numbers which are identical on the various figures represent identical or similar components.

It should also be noted that the limits of the intervals given in this application are included in the intervals.

The invention proposes a method for selecting a viscoelastic plastic interlayer comprising two outer layers and one central layer and is intended to be incorporated between two glass sheets of a glazing unit.

The method comprises the following steps:

-   -   a first component made of a viscoelastic plastic material         intended to constitute the central layer and a second component         made of a viscoelastic plastic material intended to constitute         the outer layers are provided,     -   the shear modulus G′ of the first component and of the second         component are measured by means of a viscoanalyzer,     -   the material of the second component is only selected if its         shear modulus G′ is greater than or equal to 3×10⁷ Pa at 20° C.         and for a frequency range between 100 Hz and 240 Hz; and     -   the thickness h of the first component is set so that h is less         than or equal to 0.3 mm and so that the shear parameter g=G′/h,         G′ being the shear modulus, is between 8×10⁸ Pa/m and 2.67×10⁹         Pa/m at 20° C. and for a frequency range between 100 Hz and 240         Hz.

The frequency range between 100 Hz and 240 Hz includes the four first natural frequencies of a laminated windshield, and in particular the second and third natural frequencies, as the inventors have been able to measure them by vehicle tests.

Furthermore, the inventors have demonstrated that the material which fulfills the conditions of h and g described above makes it possible to optimize the damping of the first natural frequencies of a laminated windshield, in particular of the second and third natural frequencies of the windshield comprising two glass sheets and an interlayer selected as described above.

Specifically, as will be seen further on, in particular during the study of FIG. 1, the inventors have shown that these values of g between 8×10⁸ Pa/m and 2.67×10⁹ Pa/m for a frequency range between 100 Hz and 240 Hz, combined with values of h of less than or equal to 0.3 mm, make it possible to optimize the acoustic damping of frequencies between 100 Hz and 240 Hz.

If the material of the first component is such that its shear parameter g cannot be between 8.00×10⁸ Pa/m and 2.67×10⁹ Pa/m for a thickness of less than or equal to 0.3 mm, the material is not selected for producing the interlayer.

Furthermore, limiting the thickness of the layer of viscoelastic plastic material to 0.3 mm makes it possible to limit the material used for manufacturing the interlayer, which makes it possible to limit the weight of the glazing unit in which the interlayer is intended to be incorporated in order to form a windshield. This makes the windshield easier to handle and reduces the fuel consumption of a vehicle equipped with such a windshield.

Finally, in order for the central layer to vibrate correctly, it is necessary for the outer layers to be more rigid than the central layer, which is achieved with the elasticity conditions as defined.

The invention also relates to a viscoelastic plastic interlayer intended to be incorporated between two glass sheets of a glazing unit in order to give it vibroacoustic damping properties, the interlayer comprising:

-   -   two outer layers made of a viscoelastic plastic material, the         shear modulus G′ of which is greater than or equal to 3×10⁷ Pa         at 20° C. and for a frequency range between 100 Hz and 240 Hz,     -   a central layer having improved vibroacoustic damping         properties, of thickness h, so that h is less than or equal to         0.3 mm and so that the shear parameter g=G′/h of the central         layer, G′ being the shear modulus, is between 8×10⁸ Pa/m and         2.67×10⁹ Pa/m at 20° C. for a frequency range between 100 Hz and         240 Hz.

The interlayer is obtained by the selection method described above.

The interlayer makes it possible to optimize the damping of the first natural frequencies of a laminated windshield, in particular of the second and third natural frequencies of the windshield, without weighing the windshield down, as explained above, the laminated windshield comprising two glass sheets and the interlayer incorporated between the glass sheets.

The interlayer is intended to be integrated into a glazing unit. The glazing unit is intended to be used in a vehicle, in particular as a windshield.

The invention also relates to a glazing unit comprising such an interlayer.

The invention also relates to a vehicle comprising such a glazing unit.

FIG. 2 represents a cross-sectional view of a glazing unit according to the invention.

The glazing unit comprises two glass sheets 1, 2 between which the interlayer is inserted. The bonding of the interlayer to the glass sheets is carried out by known means, for example by stacking the glass sheets and the interlayer and passing the assembly into an autoclave.

The glass sheet 1 of the glazing unit is intended to be turned toward the outside of the vehicle whereas the glass sheet 2 is intended to be turned toward the inside of the vehicle. The glass sheet 1 is thicker than the glass sheet 2 so that the glazing unit has a better protection against external attacks (adverse weather conditions, gravel spray, etc.). Indeed, the thicker the glass is, the better its mechanical strength. However, the thicker the glass is, the heavier it is. It is therefore necessary to find a compromise between the mechanical strength and the weight of the glazing unit. Thus, the thickness of the glass sheet 1 is, for example between 1.4 mm and 2.1 mm and the thickness of the glass sheet 2 is, for example, between 1.1 mm and 2.1 mm.

In existing glazing units, the thickness of the glass sheet 1 is, in general, 2.1 mm and the thickness of the glass sheet 2 is, in general, 1.6 mm.

Preferably, according to the invention, the thickness of the glass sheet 1 is 1.8 mm and the thickness of the glass sheet 2 is 1.4 mm in order to limit the weight of the windshield, which makes it possible to handle it more easily and to economize on material. This also makes it possible to reduce the fuel consumption of a vehicle equipped with such a windshield. These reduced thicknesses of the glass sheets are possible without loss of the acoustic or mechanical performances compared to the existing glazing units, as will be seen further on.

The invention may also apply to windshields, for which the thickness of the glass sheet 1 is 1.6 mm and the thickness of the glass sheet 2 is 1.2 mm, or else to windshields for which the thickness of the glass sheet 1 is 1.4 mm and the thickness of the glass sheet 2 is 1.1 mm.

The interlayer is constituted of two outer layers 4, 5 between which a central layer 3 is inserted.

The outer layers 4, 5 have a shear modulus G′ greater than or equal to 3×10⁷ Pa at 20° C. and for a frequency range between 100 Hz and 240 Hz. Preferably, the shear modulus G′ of the outer layers is between 10⁸ Pa and 2×10⁸ Pa in order to further improve the vibroacoustic damping of the central layer. The outer layers are, for example, made of polyvinyl butyral (PVB).

The central layer 3 is chosen so that its thickness h is less than or equal to 0.3 mm and so that the shear parameter g=G′/h of the central layer, G′ being the shear modulus, is between 8.00×10⁸ Pa/m and 2.67×10⁹ Pa/m at 20° C. and for a frequency range between 100 Hz and 240 Hz. The central layer 3 thus has optimized acoustic performances for the frequencies between 100 Hz and 240 Hz. The central layer 3 is, for example, made of a hot-melt pressure sensitive adhesive (Hot Melt PSA), for example made of SikaMelt 9285 HT from the company Sika.

In order to reduce the weight of the glazing unit as much as possible, it is preferred that the thickness of the central layer 3 is less than or equal to 0.1 mm.

The central layer is then chosen so that h is less than or equal to 0.1 mm and so that G′/h is between 1.33×10⁹ Pa/m and 2.00×10⁹ Pa/m at 20° C. and for a frequency range between 100 Hz and 240 Hz, which makes it possible to optimize the vibroacoustic damping even more.

The acoustic properties of the central layer 3 are also defined by its loss factor tan δ. The central layer 3 is chosen so that its loss factor is greater than 0.6 at 20° C. and for a frequency range between 100 Hz and 240 Hz, in order to allow satisfactory damping.

The shear modulus G′ and the loss factor tan δ are measured using a viscoanalyzer.

The glass sheets contribute to the vibroacoustic properties of a glazing unit. The thicker the glass sheets, the higher the excitation must be in order to make the glazing unit vibrate. However, the optimization of the acoustic performances of the interlayer for frequencies between 100 Hz and 240 Hz makes it possible to reduce the thickness of the glass sheets down to 1.8 mm, even down to 1.6 mm or even 1.4 mm, for the glass sheet intended to be turned toward the outside of the vehicle and down to 1.4 mm, even down to 1.2 mm or even 1.1 mm, for the glass sheet intended to be turned toward the inside of the vehicle, without acoustic loss relative to existing glazing units. Typically, the existing windshield glazing units have glass sheets with respective thicknesses of 2.1 mm and 1.6 mm, inserted between which is an interlayer comprising a layer of viscoelastic plastic material having a thickness of 0.1 mm with a shear modulus G′ between 10⁶ and 2.10⁷ Pa and a loss factor greater than 0.6 for a temperature of 20° C. and a frequency of 50 Hz. The reduction in the thickness of the glass sheets allows a reduction in the weight of the glazing unit, therefore improved handleability and also a reduction in material, and a reduction in the fuel consumption of a vehicle equipped with such a windshield.

The two outer layers 4, 5 have the same thickness e. The thickness of each outer layer 4, 5 is determined both so that the total thickness (2e+h) of the interlayer is at most 0.86 mm, that is to say the thickness of the interlayers in existing acoustic windshields, and so that the mechanical properties of the outer layers are sufficient to satisfy the mechanical strength standards defined for motor vehicles, in particular United Nations Regulation No. 43 (referred to as Regulation R43) concerning the hard-impact strength standards. The United Nations Regulation R43 relates to the adoption of uniform technical specifications relating to the approval of safety glazing and of the installation of this glazing in road vehicles. Specifically, it is desirable for the total thickness of the interlayer to be as low as possible for reasons of weight of the windshield on the one hand, which makes it possible to reduce the fuel consumption of a vehicle equipped with such a windshield, and saving of material on the other hand.

In order to do this, the thickness e is minimized by taking into account both the adhesion of an interlayer constituted of the material of the outer layers with respect to the two glass sheets and the tear strength of the material of the outer layers.

The adhesion is evaluated on the basis of the test and calculation method described in patent application EP-A-1 495 305, which is repeated hereinbelow.

Firstly, a torsional stress is exerted on a specimen of a laminated glazing unit composed of two glass sheets and of an interlayer constituted of the material of the outer layers, until the onset of debonding of the interlayer constituted of the material of the outer layers relative to at least one of the glass sheets. In practice, the test is carried out on a round specimen 50 of glazing, having a radius r equal to 10 mm, for example using a torsion device 500 of known type, illustrated in FIG. 6.

The device 500 comprises three jaws 51, 52, 53, a pulley 54 of radius R equal to 100 mm connected to a drive chain 55 of vertical axis. The jaws are each in the form of 120° circular arcs, so as to grip the entire specimen. The surface coating of the jaws is made of a material that is mechanically compatible with the glass, for example aluminum, Teflon®, or polyethylene.

One of the jaws is held fixed against a frame, whilst another jaw is fastened to the pulley 54 that is intended to rotate in order to exert a torsion on the specimen. The pulley 54 is made to rotate by the movement of the chain 55 connected to the pulley. The chain is pulled at a minimum constant speed of 35 to 50 mm/min.

A force sensor is used to measure the force F needed to initiate the onset of debonding of the interlayer constituted of the material of the outer layers as the specimen is being twisted. From this it is then possible to deduce, by calculation, the adhesive shear strength using the known formula:

${\tau = \frac{2\;{FR}}{\pi\; r^{3}}},$ where F is the force needed to initiate the onset of debonding of the interlayer constituted of the material of the outer layers, R is the radius of the pulley 54 and r is the radius of the specimen.

As explained in application EP-A-1 495 305, the device 500 is however bulky, which means that the tests must be carried out in the laboratory. The device 500 is therefore ill-suited to measurements of the “process indicator” type on a laminated glazing manufacturing line. However, for the manufacture of laminated glazing units, although the composition of the polymeric interlayer is designed to meet the strength values set by the invention, poor adhesion of the interlayer may nevertheless occur in the finished product due to parameters associated with the method for manufacturing the glazing unit, such as the storage conditions of the interlayer, the glass washing quality, or else the temperature and pressure forces in the course of the calendering step during assembly of the glass and the interlayer, which influence the quality of the bonding.

In order to carry out measurements during monitoring of the manufacture close to the manufacturing line so that it becomes possible to quickly intervene in the process in response to poor measured strength values, it is possible, as a variant, to use another measuring device 600 which, advantageously, is more compact and easily transportable. This device 600, represented in FIG. 7, is miniaturized to around 60 cm by 20 cm and comprises two three-jaw systems 60 and 61, a rotating shaft 62, a motor 63 for rotating the shaft, a torque meter 64 and a box 65 housing the computing elements.

The round specimen of the laminated glazing unit, composed of two glass sheets and of an interlayer constituted of the material of the outer layers, is intended to be sandwiched between the two jaw systems 60 and 61, one of the systems 60 being fixed while the other is capable of being moved and rotated by means of its connection to the shaft 62. The torque meter is placed between the motor and the moveable jaw system 61. The rotation speed of the shaft depends on the thickness of the interlayer. By way of example, for an interlayer constituted of the material of the outer layers having a thickness of 0.76 mm, the rotation is of the order of 0.08 rpm.

The system 61 rotates and when the measured torque reverses, the initiation of debonding of the interlayer constituted of the material of the outer layers has taken place. The torque meter is connected to the computing elements of the box 65, which includes a display part on which the value of the adhesive strength τ can be read directly.

Whichever device is used, with a view to having a detailed understanding of the scatter in the value of the adhesive strength τ, it is preferred to repeat the test on several specimens, for example a minimum number of five specimens, and to calculate an average of the strength τ together with its standard deviation.

It is verified that the value of the adhesive strength τ obtained is within a range of admissible values in order for any laminated glazing unit to withstand the stresses corresponding to Regulation R43 (hard-impact strength). This range of admissible values is determined experimentally, from normalized mechanical strength tests defined in Regulation R43, which were carried out on laminated glazing units of different compositions.

The range of admissible values for Regulation R43, within which any value of the adhesive strength τ is suitable for satisfying the criterion of adhesion, is all of the values below 5 MPa. Preferably, the range of admissible values of the adhesive strength τ for Regulation R43 is between 2 MPa and 5 MPa, the lower limit of this range of values being determined in order to ensure a good transparency of the glazing unit, independently of the mechanical strength considerations of the glazing unit.

Once it has been verified that the adhesive strength τ of the interlayer constituted of the material of the outer layers lies within the aforementioned range of admissible values, the actual sizing of the outer layers of the interlayer is carried out.

The graphs from FIGS. 4 and 5 illustrate two possible approaches for sizing these outer layers so that they satisfy the requirements of Regulation R43 for hard-impact strength.

According to a first approach, which corresponds to FIG. 4, a glazing unit is sized with a total thickness e_(g-dim) of glass sheets in the laminated glazing unit that is fixed and equal to 4.2 mm, which corresponds, for example, to a thickness of each glass sheet of 2.1 mm, and an interlayer constituted of the material of the outer layers, of specific composition c_(i), for example a PVB-based interlayer. The central layer is ignored for the determination of the thickness of the outer layers.

In this case, with a view to sizing the interlayer constituted of the material of the outer layers, firstly a curve C₃, seen in FIG. 4 and representative of the minimum interlayer tear strength J_(c-min) required for any laminated glazing unit to withstand the stresses corresponding to Regulation R43, is plotted as a function of the interlayer thickness e_(i) of any laminated glazing unit, this curve being established for a glass sheet thickness e_(g) equal to 4.2 mm. In practice, the curve C₃ is obtained from normalized tests of mechanical strength defined in Regulation R43, carried out on laminated glazing units of different compositions in terms of thickness of their interlayer.

Next, a reference laminated glazing unit is identified which withstands the stresses corresponding to Regulation R43, with a glass substrate thickness equal to 4.2 mm and which comprises an interlayer having the specific composition c_(i). An example of such a reference laminated glazing unit is the known glazing unit 2.1/0.76/2.1, which comprises two glass substrates each having a thickness of 2.1 mm and two interlayer layers having a standard thickness of 0.38 mm and having the composition c_(i), which corresponds to an interlayer thickness e_(i-ref) of the reference laminated glazing unit equal to 0.76 mm. The resistance of this reference glazing unit to the stresses corresponding to Regulation R43 is verified by a normalized mechanical strength test, in this example with an impact drop height of 4 m.

The tear strength J_(c-ref) of the interlayer of the reference glazing unit 2.1/0.76/2.1 is then determined by the Tielking method.

This method, developed by M. Tielking, consists in evaluating the tear strength of a material based on a method of calculating the crack root energy J. The Tielking method is explained in particular in patent applications EP-A-1 151 855 and EP-A-1 495 305, repeated, in part, below.

The tear strength of an interlayer is a characteristic intrinsic to the material constituting it. It is characterized by an energy value representative of the energy needed to propagate a crack initiated in the material. This energy, known as the critical energy J_(c), is different for each type of material and is independent of the thickness of the interlayer film.

The tear strength or critical energy J_(c) is given in a known manner by an energy method based on the Rice integral J, which defines the energy localized at the root of a crack in a film subjected to very high stresses at the location of a crack. It is written in the simplified mathematical form:

${J = {{- \frac{1}{e_{1}}}\left( \frac{\partial U}{\partial a} \right)}},$ for a given stretch d of the specimen tested which will hereafter be called the displacement d, and in which:

-   -   e₁ is the thickness of the specimen;     -   a is the length of the crack; and     -   U is the potential energy of the specimen.

The experimental device for determining the tear strength is illustrated in FIG. 8. Tensile tests using a tension-compression machine 700 are carried out on several specimens Ex_(n), for example four specimens, of the same material and with a surface area equal to 100 mm² (50 mm in length by 20 mm in width). Each specimen is notched on its sides and perpendicular to the tensile force, having a different crack length a for each specimen Ex_(n), corresponding to 5, 8, 12 and 15 mm respectively.

Each specimen Ex_(n) is stretched perpendicular to the cracks 20 at a stretch rate of 100 mm/min and over a given stretch length or distance d and in an environment in which the temperature is 20° C.

According to the method described in detail in EP-A-1 495 305, it is possible to establish a curve of variation of the crack root energy J as a function of the stretch d undergone by the specimen (not represented). Using a video camera which displays the propagation of the crack 70, it is then detected at which displacement d_(c) the propagation of the crack in the specimen starts. Then, from the curve J(d) the value of the critical energy J_(c) for initialization of the tearing of the specimen, corresponding to the displacement d_(c), is deduced. It is at this critical value J_(c) that the material tears and that it is consequently mechanically damaged with respect to the required mechanical function.

The value of the tear strength J_(c-ref) measured for the interlayer of composition c_(i) of the reference glazing unit 2.1/0.76/2.1 is 31 000 J/m².

Using the curve C₃ from FIG. 4, the minimum required interlayer thickness e_(i-min) corresponding to a minimum required interlayer tear strength value J_(c-min) equal to the tear strength of the interlayer of the reference laminated glazing unit J_(c-ref) is then deduced. As shown on curve C₃, the minimum required interlayer thickness e_(i-min) is equal to 0.45 mm.

Thus, it is possible to size the laminated glazing unit constituted of two glass sheets having a thickness of 2.1 mm each and an interlayer constituted of the material of the outer layers with a thickness e_(i-dim) of the interlayer greater than or equal to the minimum required interlayer thickness e_(i-min)=0.45 mm. Preferably, the interlayer thickness e_(i-dim) of the laminated glazing unit is greater than the minimum required interlayer thickness value e_(i-min) only within a limit of 20% above this value, that is to say, in the preceding example, e_(i-dim) is preferably such that 0.45 mm≦e_(i-dim)≦0.55 mm.

Thus a laminated glazing unit is obtained that satisfies the requirements of Regulation R43, which comprises two glass sheets 1, 2 having a thickness of 2.1 mm and an interlayer with a central layer 3 and two outer layers 4, 5, each of the outer layers having a thickness between 0.225 mm and 0.275 mm.

The curve C₃ can be plotted for other glass sheet values, for example 1.8 mm and 1.4 mm. Each of the outer layers then has a thickness between 0.2 mm and 0.37 mm.

According to a second possible approach for sizing the outer layers of the interlayer, which corresponds to FIG. 5, a laminated glazing unit is sized without arbitrarily setting the thickness of the glass sheets, the glazing unit comprising an interlayer constituted of the material of the outer layers.

A three-dimensional graph C₄ is plotted, seen in FIG. 5, that is representative of the minimum interlayer tear strength J_(c-min) required in order for any laminated glazing unit to withstand the stresses corresponding to Regulation R43, as a function both of the thickness e_(i) of the interlayer constituted of the material of the outer layers of any laminated glazing unit and of the thickness e_(g) of the glass sheets of any laminated glazing unit. The graph C₄ from FIG. 5 is obtained from normalized mechanical strength tests defined in Regulation R43, carried out on laminated glazing units having different compositions in terms of interlayer thickness and of substrate thickness.

The tear strength J_(c-ref) of a reference laminated glazing unit, which withstands the stresses corresponding to Regulation R43 and which comprises an interlayer having the specific composition c_(i), is then determined.

The known laminated glazing unit 2.1/0.76/2.1 described above may, for example, act as the reference laminated glazing unit, as can the laminated glazing unit 2.1/0.76/1.8 which is also known, which comprises two glass sheets having respective thicknesses of 2.1 mm and 1.8 mm and two layers of interlayer constituted of the material of the outer layers of standard thickness 0.38 mm having the composition c_(i), which corresponds to an interlayer thickness e_(i-ref) equal to 0.76 mm. The tear strength J_(c-ref) of one or the other reference glazing unit under stresses corresponding to Regulation R43 is evaluated as before by the Tielking method.

Using the graph C₄, a combination of optimum values e_(i-opt), e_(g-opt) of the thickness of the interlayer constituted of the material of the outer layers and of the thickness of glass sheets which corresponds to a minimum required interlayer tear strength value J_(c-min) equal to the tear strength of the interlayer of the reference laminated glazing unit J_(c-ref) is then deduced. For example, when starting from the reference glazing 2.1/0.76/2.1, which corresponds to a value of the tear strength J_(c-ref) of 31 000 J/m², the points that provide a combination of optimum values e_(i-opt), e_(g-opt) are the points of the area, or surface, of the graph C₄ which correspond to a J_(c-min) value of 31 000 J/m². On this subject, it is noted that each of the optimum values e_(i-opt) or e_(g-opt) is not necessarily, individually, a minimum value of the thickness of the interlayer constituted of the material of the outer layers or a minimum value of the thickness of the glass sheets. It is the combination of the values e_(i-opt) and e_(g-opt) which results in a minimized value of the overall thickness of the laminated glazing unit.

As is seen in the graph C₄, the combination of values e_(i)=0.5 mm and e_(g)=1.8 mm/1.4 mm is a combination of values greater than or equal to a combination of optimum values.

Hence, it is possible to size the laminated glazing unit with a thickness e_(i-dim) of the interlayer constituted of the material of the outer layers that is greater than or equal to 0.5 mm and thicknesses e_(g-dim) of the glass sheets of 1.8 mm and 1.4 mm, respectively, this laminated glazing unit satisfying the requirements of Regulation R43.

The interlayer of composition c_(i) considered in the above examples has average performances from the point of view of its tear strength and the tear strength levels of compositions of better-performing interlayers currently known make it possible to envisage combinations of optimum values e_(i-opt), e_(g-opt) that are further reduced compared to the values given previously.

In particular, for a laminated glazing unit suitable for withstanding the stresses corresponding to Regulation R43 and that comprises two glass substrates having respective thicknesses of 1.8 mm and 1.4 mm, the minimum required interlayer thickness e_(i-min) may drop to around 0.4 mm. Hence, the optimized interlayer thickness e_(i-dim) of a laminated glazing unit corresponding to the requirements of Regulation R43 and that comprises two glass substrates having respective thicknesses of 1.8 mm and 1.4 mm is, generally, such that 0.4 mm≦e_(i-dim)≦0.74 mm, the lower limit of this interval corresponding to the interlayer compositions that have high performances from the point of view of their tear strength.

Thus, a laminated glazing unit is obtained that satisfies the requirements of Regulation R43 and which comprises two glass sheets 1, 2 having respective thicknesses of 1.8 mm and 1.4 mm and an interlayer with a central layer 3 and two outer layers 4, 5, each of the outer layers having a thickness between 0.2 mm and 0.37 mm.

Thus, the thickness e of each outer layer 4, 5 is set so that they provide sufficient mechanical properties to the glazing unit, namely mechanical properties that satisfy Regulation R43. This is carried out in the following manner:

-   -   it is verified that the adhesion of the material of the outer         layers is compatible with the requirements of Regulation R43 by         twisting a specimen of the interlayer constituted of the         material of the outer layers bonded to two glass sheets, by         measuring the torsional force (F) at which separation of the         interlayer constituted of the material of the outer layers from         the glass sheets is initiated, and by calculating from this         force (F) the corresponding adhesive shear strength (τ), and         this value of the adhesive strength (τ) is compared with a range         of admissible values so that any laminated glazing unit         withstands the stresses corresponding to Regulation R43; and     -   the thickness e of each of the outer layers so that they satisfy         the requirements of Regulation R43 is set in the following         manner:         -   a reference laminated glazing unit is identified that             withstands the stresses corresponding to Regulation R43 and             which comprises two sheets of glass and an interlayer             constituted of the material of the outer layers;         -   the tear strength (J_(c-ref)) of the interlayer of the             reference laminated glazing unit, the thickness (e_(i-ref))             of the interlayer of the reference laminated glazing unit             and the thickness (e_(g-ref)) of the glass sheets of the             reference laminated glazing unit are determined;         -   using a graph (C₃) representative of the minimum interlayer             tear strength (J_(c-min)) required so that any laminated             glazing unit withstands the stresses corresponding to             Regulation R43, as a function of the interlayer thickness             (e_(i)) of any laminated glazing unit, this graph being             established for a substrate thickness of any laminated             glazing unit equal to the substrate thickness in the             reference laminated glazing unit (e_(g)=e_(g-ref)), the             minimum required interlayer thickness (e_(i-min)) is deduced             which corresponds to a minimum required interlayer tear             strength value equal to the tear strength of the interlayer             of the reference laminated glazing unit             (J_(c-min)=J_(c-ref)); and         -   the thickness e of each outer layer is set so that e is             greater than or equal to said optimum interlayer thickness             value (e_(i-opt)).

As a variant, both the thickness e of each outer layer 4, 5 and the thickness of the glass sheets are set so that the assembly provides sufficient mechanical properties to the glazing unit, namely mechanical properties that satisfy Regulation R43. This is carried out in the following manner:

-   -   it is verified that the adhesion of the material of the outer         layers is compatible with the requirements of Regulation R43 by         twisting a specimen of the interlayer constituted of the         material of the outer layers bonded to two glass sheets, by         measuring the torsional force (F) at which separation of the         interlayer constituted of the material of the outer layers from         the glass sheets is initiated, and by calculating from this         force (F) the corresponding adhesive shear strength (τ), and         this value of the adhesive strength (τ) is compared with a range         of admissible values so that any laminated glazing unit         withstands the stresses corresponding to Regulation R43; and     -   the thickness e of each of the outer layers and the total         thickness of the glass sheets so that they satisfy the         requirements of Regulation R43 is set in the following manner:         -   a reference laminated glazing unit is identified that             withstands the stresses corresponding to Regulation R43 and             which comprises two sheets of glass and an interlayer             constituted of the material of the outer layers;         -   the tear strength (J_(c-ref)) of the interlayer of the             reference laminated glazing unit, the thickness (2e_(−ref))             of the interlayer of the reference laminated glazing unit             and the thickness (e_(g-ref)) of the glass sheets of the             reference laminated glazing unit are determined;         -   using a graph (C₄) representative of the minimum interlayer             tear strength (J_(c-min)) required so that any laminated             glazing unit withstands the stresses corresponding to             Regulation R43, as a function of the interlayer thickness             (e_(i)) of any laminated glazing unit, and of the thickness             of glass sheets (e_(g)) of any laminated glazing unit, a             combination of optimum values (e_(i-opt), e_(g-opt)) of the             interlayer thickness and of the thickness of glass sheets is             deduced which corresponds to a minimum required interlayer             tear strength value equal to the tear strength of the             interlayer of the reference laminated glazing unit             (J_(c-min)=J_(c-ref)); and         -   the thickness e of each outer layer is set so that e is             greater than or equal to said optimum interlayer thickness             value (e_(i-opt)) and the thickness of the glass sheets             (e_(g-dim)) is set greater than or equal to said optimum             glass sheet thickness value (e_(g-opt)).     -   The total thickness of the interlayer is preferably less than or         equal to 0.86 mm.

FIG. 3 represents a cross-sectional view of a glazing unit according to the invention, according to another embodiment.

The glass sheets 1, 2 and the outer layers 4, 5 of the interlayer are identical with respect to those from FIG. 2.

The central layer 3 comprises two damping films 6, 7. The damping film 6 is constituted of a viscoelastic plastic material A, which is different from the viscoelastic material B of which the damping film 7 is constituted. According to one embodiment, the materials A, B are coextruded. As a variant, they are laminated.

The damping films 6, 7 constituting the central layer 3 make it possible to optimize the damping of the acoustic vibrations over different temperature ranges. Thus, the film 6 optimally damps acoustic vibrations over a first temperature range and the film 7 optimally damps acoustic vibrations over a second temperature range. The first and second temperature ranges do not overlap. This makes it possible to optimize the damping of frequencies between 100 Hz and 240 Hz over a wider temperature range.

In order to do this, the material A is chosen to have, over a temperature range t_(A) and for a frequency between 100 Hz and 240 Hz, a loss factor greater than 0.6 and a shear parameter between 8×10⁸ Pa/m and 2.67×10⁹ Pa/m. Likewise, the material B is chosen to have, over a temperature range t_(B) and for a frequency between 100 Hz and 240 Hz, a loss factor greater than 0.6 and a shear parameter between 8×10⁸ Pa/m and 2.67×10⁹ Pa/m.

Furthermore, the film 6 or 7 which has the highest loss factor for a given temperature range included within the temperature range respectively t_(A) or t_(B), has an equivalent shear parameter g_(AorB eq)=h×g_(AorB), g_(AorB) being the shear parameter of the material respectively A or B respectively constituting the film 6 or 7 and h being the thickness of the central layer, which is much lower than the equivalent shear parameter of the other film 7 or 6 respectively for said given temperature range.

The given temperature range is the first temperature or the second temperature range in which the film 6 or 7 respectively optimally damps the acoustic vibrations at frequencies between 100 Hz and 240 Hz.

Thus, it is not only necessary for the loss factor of each of the films 6, 7 to be greater than 0.6 and for the shear parameter of the material of each of the films to be between 8×10⁸ Pa/m and 2.67×10⁹ Pa/m within its corresponding temperature range, but it is necessary for the most damping film (highest tan δ) in the corresponding temperature range to have an equivalent shear modulus that is much lower than the other film. In this way, the interlayer will have a behavior similar to the most damped film over each temperature range. The interlayer will thus provide optimum damping over each of the temperature ranges for which each of the films constituting the interlayer plays an optimum damping role.

Proof will now be provided that the damping of the frequencies between 100 Hz and 240 Hz is indeed optimal for the ranges of g described above.

FIG. 1 represents a curve of the modal damping of a laminated windshield as a function of the shear parameter of a central layer of the interlayer of the laminated windshield for frequencies between 100 Hz and 240 Hz at 20° C. This curve has been calculated by a finite element method.

The calculation was carried out for a windshield constituted of two glass sheets having respective thicknesses of 1.4 mm and 1.8 mm, between which an interlayer made of a viscoelastic plastic material is incorporated. The interlayer comprises three layers: a central layer incorporated between two outer layers, the total thickness of the interlayer being set at 0.76 mm. The curve from FIG. 1 is plotted for various thicknesses of the central layer between 0.05 mm and 0.3 mm.

As is shown in FIG. 1, the range 20 of the shear parameter g between 8.00×10⁸ Pa/m and 2.67×10⁹ Pa/m for a central layer 3 having a thickness h less than or equal to 0.3 mm, at 20° C. and for a frequency range between 100 Hz and 240 Hz enables a modal damping greater than or equal to 0.34, which is therefore very good since the maximum of the curve is at 0.37 for a central layer thickness of 0.3 mm.

The range 10 of the shear parameter g between 1.33×10⁹ Pa/m and 2.00×10⁹ Pa/m for a central layer 3 having a thickness h less than or equal to 0.1 mm, at 20° C. and for a frequency range between 100 Hz and 240 Hz enables a modal damping greater than or equal to 0.34, which is therefore an optimized damping. Specifically, the modal damping curve reaches a peak at 0.345 for a central layer thickness of 0.1 mm and at 0.34 for a central layer thickness of 0.05 mm.

Therefore, the invention does indeed make it possible to optimize the vibroacoustic damping of the first natural frequencies of a windshield comprising the interlayer as described above.

The interlayer according to the invention also enables good vibroacoustic damping outside of the frequency range for which it is optimized. 

The invention claimed is:
 1. A viscoelastic plastic interlayer intended to be incorporated between two glass sheets of a glazing unit, the interlayer providing vibroacoustic damping properties to the glazing unit, the interlayer comprising: two outer layers made of a viscoelastic plastic material, a shear modulus G′ of the outer layers being greater than or equal to 3×10⁷ Pa at 20° C. and for a frequency range between 100 Hz and 240 Hz; a central layer having a thickness h and having vibroacoustic damping properties such that h is less than or equal to 0.3 mm and such that a shear parameter g of the central layer is between 8×10⁸ Pa/m and 2.67×10⁹ Pa/m at 20° C. and for a frequency range between 100 Hz and 240 Hz, where g=G′/h, wherein the central layer is disposed between the two outer layers.
 2. The interlayer as claimed in claim 1, wherein the thickness h of the central layer is less than or equal to 0.1 mm, such that G′/h is between 1.33×10⁹ Pa/m and 2×10⁹ Pa/m at 20° C. and for a frequency range between 100 Hz and 240 Hz.
 3. The interlayer as claimed in claim 1, wherein the central layer has a loss factor tan δ greater than 0.6.
 4. The interlayer as claimed in claim 1, wherein the outer layers have a shear modulus G′ between 10⁸ Pa and 2×10⁸ Pa at 20° C. and for a frequency range between 100 Hz and 240 Hz.
 5. The interlayer as claimed in claim 1, wherein each of the outer layers has a thickness e such that adhesion of the material of the outer layers is compatible with requirements of Regulation R43, the adhesion being determined by twisting a specimen of interlayer constituted of the material of the outer layers bonded to two glass sheets, by measuring torsional force (F) for which separation of the interlayer constituted of the material of the outer layers from the glass sheets is initiated, and by calculating, from the torsional force (F), a corresponding adhesive shear strength (τ), and then by comparing the calculated corresponding adhesive shear strength (τ) to a range of admissible values so that any laminated glazing unit withstands stresses corresponding to Regulation R43, and wherein the thickness e of each of the outer layers is set so as to satisfy the requirements of Regulation R43, the thickness e being determined in the following manner: a reference laminated glazing unit is identified which withstands the stresses corresponding to Regulation R43 and which comprises two glass sheets and an interlayer constituted of the material of the outer layers, a tear strength (J_(c-ref)) of the interlayer of the reference laminated glazing unit, a thickness (e_(i-ref)) of the interlayer of the reference laminated glazing unit, and a thickness (e_(g-ref)) of the glass sheets of the reference laminated glazing unit are determined; using a graph (C₃) representative of the minimum interlayer tear strength (J_(c-min)) required so that any laminated glazing unit withstands the stresses corresponding to Regulation R43, as a function of the interlayer thickness (e_(i)) of any laminated glazing unit, this graph being established for a substrate thickness of any laminated glazing unit equal to the substrate thickness in the reference laminated glazing unit (e_(g)=e_(g-ref)), the minimum required interlayer thickness (e_(i-min)) is deduced which corresponds to a minimum required interlayer tear strength value equal to the tear strength of the interlayer of the reference laminated glazing unit (J_(c-min)=Jc-ref); and the thickness e of each outer layer is set so that e is greater than or equal to an optimum interlayer thickness value (e_(i-opt)).
 6. The interlayer as claimed in claim 1, wherein the central layer comprises: a damping film made of a viscoelastic plastic material A of thickness h_(A) and with a shear parameter g_(A), a damping film made of a viscoelastic plastic material B of thickness h_(B) and with a shear parameter g_(B), the materials A and B each having, over a respective temperature range t_(A) and t_(B) and for a frequency between 100 Hz and 240 Hz, a loss factor greater than 0.6 and a shear parameter between 8×10⁸ Pa/m and 2.67×10⁹ Pa/m, and the damping film (having material A or B) which has the highest loss factor, for a given range of temperatures included within the temperature range t_(A) or t_(B) respectively, has an equivalent shear parameter g_(A or B eq)=g_(A or B)×h, g_(A or B) being the shear parameter of the material constituting the film (respectively having material A or B) and h being the thickness of the central layer, which is less than an equivalent shear parameter of the other damping film (having material B or A respectively) for said temperature range.
 7. A windshield comprising: two glass sheets; and the interlayer as claimed in claim 1, the interlayer being disposed between the glass sheets for vibroacoustic damping of the second and third natural frequencies of the windshield.
 8. The interlayer as claimed in claim 2, wherein the central layer has a loss factor tan δ greater than 0.6.
 9. The interlayer as claimed in claim 2, wherein the outer layers have a shear modulus G′ between 10⁸ Pa and 2×10⁸ Pa at 20° C. and for a frequency range between 100 Hz and 240 Hz.
 10. The interlayer as claimed in claim 3, wherein the outer layers have a shear modulus G′ between 10⁸ Pa and 2×10⁸ Pa at 20° C. and for a frequency range between 100 Hz and 240 Hz. 